noun - Blue Monday is a name given to a day in January (typically the third Monday of the month) publicised as the most depressing day of the year for those in the Northern Hemisphere.
The concept was first promoted as part of a 2005 press release from holiday company Sky Travel, which claimed to have calculated the date using its unique equation. This date was published in a press release under the name of Cliff Arnall, at the time a tutor at the Centre for Lifelong Learning, a Further Education centre attached to Cardiff University. Guardian columnist Dr. Ben Goldacre reported that the press release was delivered substantially pre-written to a number of academics by public relations agency Porter Novelli, who offered them money to put their names to it. The Guardian later printed a statement from Cardiff University distancing themselves from Arnall: "Cardiff University has asked us to point out that Cliff Arnall... was a former part-time tutor at the university but left in February."
The date is generally reported as falling on the third Monday in January, but also on the second or fourth Monday, or the Monday of the last full week of January. The first such date declared was 24 January in 2005 as part of a Sky Travel press release.